The invention is based on a hydraulic dual-circuit brake system with diagonal brake circuit distribution, an anti-skid or anti-lock system (ABS) and traction control (ASR) for motor vehicles, in particular passenger vehicles, defined hereinafter.
A four-channel hydraulic unit for diagonal brake circuit distribution in dual-circuit brake systems with skid protection, also known as an anti-lock system (ABS), is known from Bosch publication "Kraftfahrtechnisches Taschenbuch" [Automotive Engineering Handbook] published by VDI-Verlag, 1987, pp. 530 and 531. The ABS control valves, embodied as 3/3-way magnetic valves, have three positions. In the first, non-excited position, there is unhindered passage from the master brake cylinder to the wheel brake cylinders, so that at the onset of braking the wheel brake pressure rises. In the second position, excited with half the maximum current, the passage from the master brake cylinder to the wheel brake cylinders is interrupted, so that the wheel brake pressure is kept constant. In the third position, excited with maximum current, the wheel brake cylinders are made to communicate with the return line; the brake fluid flowing out of the wheel brake cylinders per brake circuit is pumped back into the master brake cylinder by the return pump. All the magnetic valves are triggered by an ABS electronic control in accordance with the slip detected at the wheels by means of slip sensors; in general, the magnetic valves for the wheel brake cylinders of the two rear wheels are triggered in common, so that the same brake pressure prevails in both rear wheels. In the return line, small brake fluid reservoirs are typically included, which upon pressure decrease can temporarily hold a volume of approximately 2 cm 3 cm of brake fluid per brake circuit.
German Patent 36 33 687 Al, U.S. Pat. No. 4,818,039, discloses a hydraulic dual-circuit brake system with diagonal brake circuit distribution, an anti-lock system (ABS) and traction control (ASR) for motor vehicles of the type referred to at the outset, in which a four-channel hydraulic unit of this kind is used (see FIGS. 3 and 8 of this reference). To achieve the traction control, the two pressure pistons, embodied as a tandem piston, of an additional brake cylinder are actuated via a piston rod by a spring reservoir, which is relaxed if wheel slip of at least one driven wheel occurs, the tensed restoring spring displacing the pressure pistons. At the same time, the two central valves are switched over either hydraulically (FIG. 3) or electrically (FIG. 8) and block off the two cylinder chambers of the additional brake cylinder from the master brake cylinder. By means of the two pressure pistons, brake pressure is now fed into the wheel brake cylinders of the driven wheels. The electronic control controls the control valves, and the spinning driven wheel is slowed down until it is free of slip. After that, the brake fluid fed into the wheel brake cylinder is pumped back into the cylinder chamber of the additional brake cylinder by the pumping element of the return pump. The tension of the spring reservoir is effected by a low-pressure pump, which pumps brake fluid into the reservoir chamber of the spring reservoir until such time as the spring reservoir piston has reached its terminal position and opens a switch located in the current circuit of the low-pressure pump.
A dual-circuit brake system with a "front/rear" brake circuit distribution, an anti-lock system and traction control is already known from German Patent 30 21 116 A1; in it, the return pump has a total of three separate pumping elements. Two of them serve to pump the brake fluid back out of the wheel brake cylinders into the master brake cylinder in order to reduce the brake pressure during a normal braking event, while the third pumping element assures the supply of brake pressure in traction control. To this end, the third pumping element is connected on the inlet side to the brake fluid reservoir at the master brake cylinder and on the outlet side to a pressure reservoir. The brake fluid pressure built up in the pressure reservoir is present, via a 3/2-way valve with a hydraulic control inlet, at the ABS control valve for the driven rear wheels, which here is embodied as a 3/2-way magnetic valve, this pressure being available such that it is fed into the wheel brake cylinder, in the excited brake pressure reduction position of the ABS control valve. In the braking event, the 3/2-way valve is switched over by the brake pressure generated by the master brake cylinder, such that the communication between the pressure reservoir and the control valve is blocked off. When the brake is not actuated and if slip of at least one of the driven wheels occurs, the control valve is switched over, so that the wheel brake cylinder is connected to the pressure reservoir via the 3/2-way valve. In order that only the driven wheel at which the drive slip has been sensed will be slowed down, a 2/2-way magnetic valve is incorporated between the outlet of the control valve and each wheel brake cylinder. The electronic 20 control switches over the 2/2-way magnetic valve associated with the driven wheel not exhibiting any slip. The wheel brake cylinder of this driven wheel is thus uncoupled from the pressure reservoir, so that the pressure buildup occurs only in the wheel brake cylinder of the driven wheel that is slipping.